(a) Field of the Invention
The present invention relates to a ladle stream breaker for use in a tundish to continuously cast metal slabs, especially steel slabs.
(b) Brief Description of the Prior Art
It is of common practice in any continuous casting process to use a piece of equipment called "tundish", for separating slag and other contaminants from the molten metal being cst. Such a tundish is an intermediate vessel which is positioned between a casting ladle containing the molten metal to be cast, and a casting mold which is usually vertical. The molten metal is poured from the ladle into the tundish at one end thereof. The molten metal then flows along the vessel toward the other end thereof. The length of the vessel is selected to provide a time of residence of the metal in the tundish sufficient to allow separation of the inclusions as a floating slab layer. At the other end of the tundish, the molten metal free from its inclusions flows through a vertical outlet tube into the mold from which the solidifying cast slab is continuously drawn out.
Numerous studies have been made up to now to optimize the configuration and design of the existing tundishes, in order to achieve better inclusion separation, less slab entrainment into the mold, less skulling and higher metallic yield.
The solutions that have been proposed up to now and are presently used in most of the existing tundishes to improve flotation of the inclusions, are:
(1) an increase in the size of the tundish and more particularly its length; and/or PA1 (2) the addition of generally one partition dam or weir into this tundish (see, for example, U.S. Pat. Nos. 3,814,167 and 4,125,146), PA1 (1) preventing the kinetic energy of the stream of molten metal being poured from the ladle at a given height over the tundish from being sufficiently dissipated; and PA1 (2) generating eddy currents inside the pouring box.
both in order to increase the residence time of the molten metal in the tundish.
The partition, dam or weir usually extend vertically across the tundish and stop short from its bottom wall. It divides the tundish into a first portion hereinafter called "pouring box", extending from the partition towards the one end wall of the tundish, and a second portion hereinafter called "flotation box" extending from the partition towards the other end wall of the tundish. Of course, all of the walls of the tundish and its partition that are or may be in contact with the metal to be cast, are lined with, or made from a refractory material.
In use, the metal to be cast is fed in molten state as a stream from the ladle into the pouring box, where it is subjected to great turbulences. This allows sometimes tundish to be used as an "open chemical reactor" in which it is possible to introduce additives or alloys beneath a thick metal layer in a very well mixed volume, where dissolution rate is maximized.
Then, the molten metal flows from the pouring box into the flotation box under the partition, thereby preventing any slab being formed in the pouring box from being entrained.
Thereafter, the molten metal flows along the entire length of the flotation box toward the outlet tube in a very uniform manner without any short circuit, thereby allowing optimum plug flow volume to achieve high inclusion separation before the molten metal flows out of the tundish into the casting mold.
In most industrial applications, the stream of molten metal, fed from the ladle into the pouring box of the tundish, has to be shrouded to prevent reoxydation of the molten metal being cast. Such a shrouding is usually achieved by pouring the ladle stream into the tundish through a tube of refractory material which acts as an injection nozzle.
If the use of such a pouring tube has the requested advantage of shrouding the stream, it also has the major drawbacks of:
Both of these drawbacks in turn cause some of the slag being formed in the pouring box to be entrained out of the same into the flotation box.
To tentatively solve this problem, it has already been suggested to inject an inert gas into the pouring tube to reduce the compactness of the stream and thus its depth of penetration. This solution is rather efficient in practice if the level of metal in the tundish is high and the metal flow is not too high. If, however, the level of metal is low and/or the metal flow is high, the bubbles of gas that are injected and move up around the pouring tube inside the pouring box, "push" the slag layer on top of the pouring box away from the tube wall around the same, and thus create an annular zone where the molten metal is exposed to air and subject to reoxydation.